Power electronic circuits are used to control and condition electric power. For instance, power electronic circuits may be used to convert a direct current into an alternating current, to change voltage or current magnitude, or to change the frequency of an alternating current.
An inverter is a power electronic circuit which receives a dc source signal and converts it into an ac output signal. Harmonic neutralization and pulse-width modulation techniques are used to generate the ac signal. Harmonic neutralization involves a combination of several phase-shifted square-wave inverters, each switching at the fundamental frequency. Pulse-width modulation involves switching a single inverter at a frequency several times higher than the fundamental.
Inverter switching action generates transients and spurious frequencies in a power signal, usually in the form of harmonics of the switching frequency. The switching action may also produce electromagnetic interference (EMI) which is radiated or conducted through the supply line. While the internal design of an inverter is chosen to minimize transients and spurious frequencies, it is usually necessary to filter the input or the output of the inverter.
Filters can be classified according to whether their main purpose is to improve the power waveform or to remove EMI. Filters for waveform improvement usually deal with frequencies in the audio range. EMI filters are usually concerned with frequencies of 455 kHz or higher.
Passive filters are typically used to eliminate undesirable harmonics from the inverter output. Unfortunately, passive filters do not provide continuous harmonic filtering on pulsating or randomly varying loads. This occurs because passive filters only adapt to new harmonic levels after a considerable settling delay.
Passive filters tend to be large, heavy, costly, and, in general, highly load-dependent. Consequently, passive filters frequently represent a substantial part of the total cost, weight, and size of power electronics equipment.
Active filters represent an emerging technology without many of the shortcomings associated with passive filters. The technology relies upon the theory of active-feedback filters. A feedback loop with a single energy-storage element (an inductor or capacitor) is used to minimize the difference between the actual waveform and the desired waveform.
The urgency of developing successful active power filters has recently grown in view of the increasing waveform distortion of both voltages and currents in ac power distribution systems. These distortions are largely attributable to a growing number of nonlinear loads in the electric utility power network. Typical nonlinear loads are computer controlled data processing equipment, numerical controlled machines, variable speed motor drives, robotics, medical and communication equipment.
Nonlinear loads draw square wave or pulse-like currents instead of purely sinusoidal currents drawn by conventional linear loads. As a result, nonlinear current flows through the predominantly inductive source impedance of the electric supply network. Consequently, a nonlinear load causes load harmonics and reactive power to flow back into the power source. This results unacceptable voltage harmonics and load interaction in the electric power distribution in spite of the existence of voltage regulators.
The degree of current or voltage distortion can be expressed in terms of the relative magnitudes of harmonics in the waveforms. Total Harmonic Distortion (THD) is one of the accepted standards for measuring voltage or current quality in the electric power industry.
Apart from voltage and current distortion, another related problem may arise when nonlinear loads are connected to the electric power network. In particular, when the load current contains large amounts of third or other triplen harmonics, the harmonic current tends to flow in the neutral conductor of the power system. Under these conditions, the neutral current can exceed the rated current of the neutral conductor. Since the neutral is normally designed to carry only a fraction of the line current, overheating or even electric fires can result.
As previously indicated, active filters may be used to alleviate these problems. Active filters, or active power line conditioners (APLCs), comprise one or two pulse width modulated inverters in a series, parallel, or series-parallel configuration. Series/parallel configured inverters share a common dc link, which can be a dc inductor (current link) or a dc capacitor (voltage link). It is advantageous to keep the energy stored in the dc link (capacitor voltage or inductor current) at an essentially constant value. The voltage on the dc link capacitor can be regulated by injecting a small amount of real current into the dc link. The injected current covers the switching and conduction losses inside the APLC. The link voltage control can be performed by the parallel inverter.
The basic active load current compensation with current or voltage source filters is known. FIG. 1 depicts a parallel connected current source active filter 20, and FIG. 2 depicts a parallel connected voltage source active filter 22. The load current I.sub.L consists of three components: The real current, I.sub.r, the reactive current, I.sub.q, and the ripple current, I.sub.R. The parallel connected active filter supplies the I.sub.R and I.sub.q components, and, also, a small residual "high frequency" component I.sub.hf, that flows into the parallel connected "high frequency" capacitor C.sub.hf. The parallel connected active filter is essentially a single or multi-phase inverter which is operated from an isolated current or voltage source.
The realization of the active filter requires solid state switches with intrinsic turn-off capability (transistors, IGBTs, MOSFETs, GTOs, etc.). Switch pairs P1 and P2 are alternately turned ON and/or OFF. The average voltage required in the link capacitor, V.sub.dc, of FIG. 2, is supplied by the ac source. Real power can be absorbed by introducing an appropriate amount of offset in the symmetry of the on-times in switches P1 and P2. The polarity of the offset is coordinated with the polarity of the input voltage. When switches P1 of FIG. 2 are on, a resonant current is generated between the tie inductor, Lp, the output capacitance dominated by C.sub.hf, and the difference between the dc link and ac output voltages. Conversely, when the P2 switch pair is on, the resonant current is driven by the sum of the dc link and ac output voltages. Since the dc link voltage is regulated to be larger than the peak value of the ac voltage, the voltage polarity that drives the resonant current will reverse after each complementary pole switching.
The real power necessary to maintain the selected dc link voltage magnitude, Vdc, is proportional to the average duty cycle of high-frequency pole switchings in any given half line voltage cycle. The isolated dc link voltage is regulated by a closed loop controller that affects the average pole switching symmetry. Reactive inverter currents can be produced that flow in or out of the inverter by temporary changes in the duty cycle of inverter pole switchings. The instantaneous magnitudes of inverter currents are regulated so that they provide the load compensation current requirements. For example, if a positive ripple current is detected, the on-time of P2 is increased with respect to P1. The increase results in the required net compensating ripple current flowing in the ac line. This also implies that the amplitude of Vdc must be kept higher than the highest value of the ac voltage across the load, otherwise, the instantaneous compensation capability of the active filter is impaired.
The rapid pulse width modulation switching in the active filter produces a high frequency, typically, triangular shaped current, I.sub.hf, an undesired side effect. The effect of the I.sub.hf signal is a small, superimposed saw-tooth voltage ripple on the ac voltage. With a given tie inductor value, the amplitude of the voltage ripple is inversely proportional to the pole switching (carrier) frequency and the value of C.sub.hf. The voltage ripple is filtered with a parallel capacitor C.sub.hf.
When the active power filter (20 or 22) is connected across the load, a high degree of filtering of the terminal voltage is observed. Note that the active power filter is not capable of supplying or absorbing any real power other than that which is needed to compensate for losses inside the filter itself. It will, however, readily compensate reactive currents, non-synchronous and non-theoretical harmonics, and sources with variable or unregulated frequency.
Series-parallel active power line conditioners have the advantage that they can supply and absorb real power. They are also advantageous since they provide broad power conditioning capabilities. On the other hand, these benefits are accompanied by a number of disadvantages.
One disadvantage associated with series-parallel active power line conditioners is that the series connected inverter of the APLC must include expensive surge protection circuitry. If the series inverter is rated to handle the high surge voltages, the parallel inverter must also be rated to the same high voltage, since the two inverters share a common dc voltage link. In fact, the dc link must be charged to a higher than peak ac voltage level in order to maintain current control and thus avoid false, series inverter over-current trips. To rate the inverters for these surge voltage and surge current rating requirements may not result in a commercially competitive product. Thus, it is important to develop a cost-effective APLC which complies with surge rating requirements in a different way.
The surge protective functions override the active power quality controllers in an active power line conditioner. Consequently, a protective function, while in effect, can result in a temporary compromise in the output power quality, such as: elimination of output voltage regulation, injection of load harmonics back into the source, and uncompensated input voltage harmonics applied to the load. Thus, it would be highly desirable to provide an active power line conditioner which does not rely upon surge protective functions which will compromise output power quality.